Smooth glass insulating film over interconnects on an integrated circuit

ABSTRACT

A low temperature insulating glass for use in semiconductor devices comprises a mixture of germanium, silicon, oxygen and phosphorus. In the preferred embodiment, the glass comprises a mixture of about 40% to 55% silicon dioxide (SiO2), about 55% to 40% of germanium dioxide (GeO2) and from 1% to about 5% of phosphorus pentoxide (P2O5), by mole percent.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 06/243,988 filedMar. 16, 1981, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuits and in particular to suchcircuits wherein a glass film with a smooth surface is provided on theintegrated circuit over the interconnect structure on the integratedcircuit. This invention is particularly useful when the interconnectstructure is aluminum or an alloy of aluminum but can also be usedadvantageously with other interconnect materials such as polycrystallinesilicon ("polysilicon") selectively doped to a desired conductivity, ora polysilicide.

2. Description of the Prior Art

The use of glass films overlying aluminum interconnections on integratedcircuits is well known. Unfortunately, because prior art glass filmsflow at a temperature significantly above that of the aluminum, oncesuch a glass film is formed, the topography of the surface of the glassfilm is permanently fixed. Accordingly, should it be desired to smooththe glass flim to allow a second level of interconnects to be formedover the glass film, the underlying aluminum melts. To avoid or reducethis problem, first level interconnects are often formed of somematerial other than aluminum selected to have a melting temperatureabove the flow temperature of the insulating glass formed over theinterconnect leads. This often raises the impedance of the interconnectsand slows the circuit.

SUMMARY

In accordance with this invention, a glass composition is provided whichhas a flow temperature near the melting point of aluminum. The term"flow temperature" is used here to indicate that temperature to which aglass film with a rough surface topography must be heated to producesubstantial smoothing of the topography in thirty (30) minutes. Ingeneral, higher temperatures produce faster flow. This glass is thussuitable for use as insulation on an integrated circuit containingmultiple levels of interconnects.

In accordance with this invention, a glass composition comprising amixture of silicon, germanium, oxygen and phosphorus is provided whichhas the characteristic that it flows over integrated circuitinterconnects made of a material such as aluminum or dopedpolycrystalline silicon by means of pressure coupled with temperature,temperature alone or a temperature pulse generated from a source such asa laser or a heat lamp (Xenon) to provide a smooth glass surface withoutdamaging the underlying interconnect structure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the flow characteristics for the binary glass of thisinvention wherein the silicon dioxide and the germanium dioxide arearranged in 50--50 mole percent concentration, both with and without P₂O₅ added;

FIG. 2 illustrates the etch rate (angstroms per second) of the binaryglass in a buffered HF etch as a function of the mole percentconcentration of GeO₂ in the binary SiO₂ -GeO₂ glass; and

FIG. 3 is a graph illustrating the solubility in terms of angstroms persecond of the binary SiO₂ -GeO₂ glass in water at room temperature as afunction of mole percent concentration of GeO₂ in the binary glass.

DETAILED DESCRIPTION

The glass of this invention comprises a mixture of silicon, germanium,oxygen, and phosphorus. The composition generally is in the range ofapproximately 30-70% SiO₂, 30-70% GeO₂, and 1-10% P₂ O₅, by molepercent. Of course, the actual percentages of each constituent are suchthat the total of the mole percentages of all the constituents is 100%.By selecting the composition to be in these ranges, the flow temperatureof the glass is brought low enough such that the surface of the glasscan be reflowed without damaging the interconnects (such as aluminum orpolysilicon) formed beneath the glass. The preferred range ofconstituents is 45-50% SiO₂ :50-45% GeO₂ : 1 to 5% P₂ O₅ by molepercent, but we believe constituents in the range of 40-55% SiO₂ :55-40%GeO₂ :0 to 5% P₂ O₅ by mole percent are also appropriate.

FIG. 1 illustrates the flow characteristics for the binary GeO₂ -SiO₂glass deposited in accordance with the teachings of this invention. Theglass characterized by the graph of FIG. 1 comprises an SiO₂ -GeO₂ glassin 50--50 mole percent of SiO₂ and GeO₂ both with and withoutphosphorous pentoxide (P₂ O₅). The glass with phosphorous pentoxidecontains five (5) mole percent of phosphorous pentoxide. FIG. 1illustrates that during heat treatment for thirty (30) minutes in anitrogen ambient, the glass with phosphorous pentoxide added flowsheavily at a temperature slightly under 900° C. Heavy flow of the glassis obtained without phosphorous pentoxide at a temperature of slightlyunder 1000° C. Moderate flows are obtained without phosphorous pentoxideat a temperature around 900° C. and with phosphorous pentoxide at atemperature just over 800° C. Thus phosphorus pentoxide in the glassreduces the temperature required to achieve a given flow.

FIG. 2 illustrates that the etch rate using a standard oxide etch (abuffered HF known in the art as a "500 etch") of the binary glass is aminimum when the mole concentration of GeO₂ in the binary GeO₂ -SiO₂glass is about fifty (50) percent. A change in the mole percentconcentration of GeO₂ relative to SiO₂ in either direction about thispoint results in an increase in the etch rate of this glass. The variouscurves illustrate also the effect on the etch rate of first flowing thedeposited glass at three different temperatures (1000° C. for 30 minutesin argon gas, 900° C. for 30 minutes in argon gas and 800° C. for 30minutes in argon gas). While the flowing of the glass flattens the curveof etch rate versus mole percent GeO₂ at its minimum such that theminimum etch rate occurs for a mole percent GeO₂ between about 50-70mole percent, the range of minimum etch rates still includes a molepercent of GeO₂ of about 50%.

FIG. 3 illustrates the water solubility at room temperature of thebinary GeO₂ -SiO₂ glass as a function of mole concentration of GeO₂.FIG. 3 shows that the water solubility (in terms of angstrom per secondof glass removed in water) is approximately zero beneath a mole percentconcentration of GeO₂ of 60%. Accordingly, the mole percentage of GeO₂in the binary glass should not exceed 60% and preferably, for safety,should remain somewhat beneath this percentage. When FIG. 3 is comparedto FIG. 2 giving the minimum etch rate of the binary glass, it is clearthat a mole percentage of GeO₂ of about 50% in the binary glass isoptimum because for maximum preciseness and control in the etching ofvias through the binary glass, a binary glass with minimum etch rate isdesirable. Moreover, FIG. 1 illustrates that a 50--50 mole percent SiO₂-GeO₂ binary glass has satisfactory flow characteristics for integratedcircuit structures. Thus, a binary glass of about 50--50 mole percentSiO₂ -GeO₂ is preferred for use in this invention.

In accordance with this invention, the use of a high pressure, asdisclosed in co-pending application serial number 243,990 (filed on thesame day as the parent application of this specification on an inventionof Reda Razouk entitled "Method of Inducing Flow or Densification ofPhosphosilicate Glass for Integrated Circuits" and assigned to FairchildCamera and Instrument Corporation, the assignee of this application),can be used to further lower the flow temperature of the glass. The useof pressure to accelerate flow is desirable since the temperature windowbetween the highest temperature to which an aluminum interconnect can beheated without damage (typically about 540° C. but in general dependantupon the aluminum alloy used) and the highest temperature used insubsequent processing is small. The glass flow point should be kept highenough to avoid flow of the glass during later processing for alloycycles or die attach. For this reason the natural flow of glass is madeto take place at a temperature quite close to that of the melting ofaluminum alloys when aluminum is the interconnect material. In anotherapproach using temperature pulse techniques involving lasers, atemperature difference between the glass and the underlying interconnectmaterial can be sustained and the glass can be selectively reflowed at alocalized temperature somewhat higher than the melting temperature ofthe interconnect (such as an aluminum alloy) without damaging theunderlying interconnect structure.

The glass used here is similar in its properties to phosphosilicateglasses commonly used in the prior art but flows at a much lowertemperature. While the prior art phosphosilicate glass flowed, it did soat a temperature much too high to allow it to be used over aluminum ororganic dielectrics such as polyimide (which tends to absorb water).

EXAMPLE 1

A typical composition suitable for deposition over an interconnectstructure comprising either aluminum, polysilicon or a polysilicidecomprises a binary glass consisting of forty-nine (49) mole percentSiO₂, forty-nine (49) mole percent GeO₂ and two (2) mole percent P₂ O₅(corresponding to about 3.9 weight percent P₂ O₅ in the resultingglass).

EXAMPLE 2

Typical deposition conditions using a Pyrox atmospheric CVD reactor madeby Tempress involved the flow of germane (GeH₄), silane (SiH₄), oxygen(O₂), nitrogen (N₂) and phosphine (PH₃) in the Pyrox reactor at thefollowing flow rates:

    ______________________________________                                        Constituents        Flow Rates                                                ______________________________________                                        GeH.sub.4           3.67 cc/minute                                            SiH.sub.4           7.33 cc/minute                                            O.sub.2             100 cc/minute                                             N.sub.2             2 liter/minute                                            PH.sub.3 (1% in N.sub.2)                                                                          11 cc/minute                                              ______________________________________                                    

The substrates on which the glass was deposited comprised patternedsilicon wafers held at 400° C. The binary glass deposited at a rate ofapproximately 300 angstroms per minute.

The above description is exemplary only and other embodiments of thisinvention will be obvious to those skilled in the semiconductor glassarts in view of this disclosure.

What is claimed is:
 1. An integrated circuit comprising:a semiconductormaterial; insulation formed on said semiconductor material; interconnectleads formed on said insulation and selectively brought into contactwith the underlying semiconductor material through openings in saidinsulation; and a glass layer formed on said interconnect leads, saidglass layer consisting of about 45% to 50% SiO₂, about 50% to 45% ofGeO₂ and from 1% to about 5% of P₂ O₅, by mole percent, said glass layerhaving a flow temperature sufficiently low to prevent damage to saidleads and being capable of flowing over said leads at said flowtemperature to provide a smooth glass layer surface.
 2. An integratedcircuit as in claim 1 wherein said glass layer has a substantiallysmooth surface.
 3. An integrated circuit as in claim 1, wherein saidinterconnect leads comprise aluminum.
 4. An integrated circuit as inclaim 1, wherein said interconnect leads comprise polycrystallinesilicon selectively doped to a desired conductivity.
 5. In combination alow temperature insulating glass and an interconnect structure of asemiconductor device, said insulating glass consisting of about 45% to50% SiO₂, about 50% to 45% GeO₂ and 1 to 5% of P₂ O₅, all by molepercent, wherein said glass has a flow temperature under 1000° C. forsmoothing said glass while preventing damage to said interconnectstructure and wherein said glass is deposited on said structure bychemical vapor deposition at a temperature less than the flowtemperature.
 6. In combination a low-temperature insulating glass and aninterconnect structure of a semiconductor device, said insulating glassconsisting of about 45% to 50% SiO₂, about 50% to 45% GeO₂, and about 5%P₂ O₅, all by mole percent, wherein said glass has a flow temperatureunder 1000° C. for smoothing said glass while preventing damage to saidinterconnect structure and wherein said glass is deposited on saidstructure by chemical vapor deposition.